This invention relates to apparatus for direct measurement of small values of inductance.
When developing and testing RF circuits, it is often necessary to use inductances of very low value, such as inductances in the range from 10.sup.-8 H to 3X10.sup.-6 H. Known apparatus available for measuring inductances of such low value are either expensive, time consuming, or both. The least expensive technique known for measuring the impedance of a circuit element involves the use of a Wheatstone bridge in which a known element is compared with an unknown element. For resistors, the technique is quite satisfactory using DC current to avoid the effects of reactance in the measurement circuit, but for capacitors or inductors, which require AC current for the measurement, the problem is to first balance the bridge with known elements of equal value in order to balance out the effects of reactance in the measurement circuit before substituting an unknown element for one of the known elements.
Another technique for measuring inductance or capacitance utilized in a system disclosed in U.S. Pat. No. 3,480,857 depends upon detecting the change in resonant frequency of an LC tuned circuit. Cycles of the resonant frequency are first counted up for a predetermined period with a known element in the circuit. Then the unknown element is inserted, or substituted, and cycles of the new resonant frequency are counted down for the same predetermined period. The difference is proportional to the ratio of the unknown element to the known element. This requires knowing the value of the known element to the same degree of accuracy required for the unknown element, and is subject to the accuracy and stability of the system in measuring the predetermined period for counting cycles at both frequencies.